This study is designed to elucidate the rates and mechanisms of the radical attack on molecules of biological significance. The first study is an investigation of the relative importance of reactions (A) and (B) in attack of radicals on biologically important thiols. R plus R'SH yields RH plus R'S (A); R plus R'SH yields RSH plus R' (B). This study will be carried out using steady state photolysis methods to generate R is equal to CH3 and the mechanism will be deduced from product analysis. The second study is concerned with the fate of the thiyl radicals (R'S) which are formed in reactions like (A). Rates for the reaction of R'S with a variety of compounds will be determined by direct observation of the thiyl radical decay using flash photolysis techniques. A sensitive flash system with microsecond resolution will be built to determine interferences from secondary reactions in these systems. Thiyl radical reaction rates will also be determined from measurement of the RSSR dimer anion which is formed in direct competition with other reactions of R'S.